Light-scattering sheets intended for the manufacture of microcomputer screens are known. Such a sheet consists, for example, of a transparent plastic, such as poly(methyl methacrylate) (PMMA), and is coated on one of its faces with dots of suitable size, printed by a technique such as screen printing.
During its use as a microcomputer display system, such a sheet, generally rectangular in shape, is illuminated by a light source, generally one or more fluorescent tubes placed near one or more of the edges of the panel (especially parallel to these edges), so as to illuminate it (or them) via the edge. The light rays emanating from the source are transmitted by the transparent material in the bulk of the sheet, which thus operates as a lightguide.
However, some of these rays are reflected and scattered by the printed dots, and may thus be observed from that side of the sheet which faces the user. It is these reflected and scattered rays which have the effect of making the sheet luminous.
In a microcomputer display system, such a scattering sheet is generally mounted at the rear of the liquid-crystal display device and thus makes the information displayed by the latter luminous. This scattering sheet, the associated light source(s), the frame in which the assembly is mounted and, possibly, accessory elements (such as films helping to make the scattered light more uniform, prismatic films and/or protective films) constitute a unit used in the field of assembling microcomputer flat screens, often called in the art “backlight unit”, and it is this term which will be used in the present text.
It is important, for the purpose of visual comfort of the person using a microcomputer screen, to have not only a scattering sheet ensuring a very high level of illumination (of the order of 2000 lux), which assumes an appropriate efficiency in terms of light transmitted into the bulk of the sheet, but also an intensity of the light scattered towards the user which is uniform over the entire surface of the sheet.
In addition, according to a frequent configuration for a backlight unit, the scattering sheet is illuminated, for example by 2 fluorescent tubes placed parallel to the edges corresponding to the 2 opposed sides. In such a configuration, the scattered light intensity may exhibit a certain variation from one of these sides to the other, with a reduction in the central part of the sheet. It is desirable for the visual comfort of the user that the reduction in light intensity observed in the central part of the sheet be as limited as possible.
Scattering sheets provided with screen-printed dots have drawbacks arising from the difficulty of manufacturing them, this being accompanied by a high scrap rate of off-specification products. They are also fragile, especially because of problems of adhesion of the dots to the sheet or because of the possibility of scratching when handling the said sheets.
Also known are light-scattering panels that can be used for light display systems for advertising or information purposes. These display systems generally comprise, mounted in a suitable frame, an assembly comprising a sheet (or panel) made of a transparent plastic containing particles of material able to scatter light. These sheets may be illuminated by light sources placed near the edge(s).
Thus, U.S. Pat. No. 6,107,444 discloses a transparent plastic composition comprising polyamide particles, which can be used for light display systems. The levels of illumination allowed by such a system, of the order of a few hundred lux, are, however, not high enough to illuminate a microcomputer screen.
International Application U.S. 2003119990 discloses a composition made of a transparent plastic comprising polytetrafluoroethylene (PTFE) particles, which can be used especially for flat light screens. Although the PTFE particles allow plates with a certain thickness to obtain a high scattered light intensity, the latter suffers, however, an excessive reduction in the central part of the screen.
Japanese Application JP 50-002180 discloses a thermoplastic resin composition obtained by polymerization of a monomer deriving from methyl methacrylate, which comprises from 0.2 to 20% by weight of glass particles. This composition is used to prepare a moulded product by stretching a cast sheet, the said product affording, owing to its surface roughness, advantageous optical properties beneficially used in the construction of a skylight.